The present invention relates to a pneumatic tire, in which a carcass ply is made of low twist organic cords so as to achieve a tire weight reduction or a carcass strength improvement.
In a high performance tire having an aspect ratio of not more than 80%, organic fiber cords, e.g. polyester, rayon and nylon are generally used in the carcass. In such a carcass cord, in order to provide a fatigue resistance to a compressive stress and thereby to prevent carcass cords, which are located axially outside a bead apex, from being damaged during running under a heavy load condition, the cord twist has been set at a larger value for example 40 turns/10 cm or more.
Such a high twist cord is however poor in tensile strength, and therefore is not suitable for a high performance tire used under very high pressure and very high speed conditions.
Accordingly, in order to obtain the desired tensile strength for the carcass, the diameter of the carcass cord, the cord count of a carcass ply, and/or the number of carcass plies have been increased. As a result, tire weight is undesirably increased, deteriorating the tire's running performance. On the other hand, if such parameters are decreased to reduce tire weight, the carcass strength is reduced which also deteriorates the tire's running performance.
It is therefore, an object of the present invention to provide a pneumatic radial tire, in which the above-mentioned problems are solved.